Ductile cobalt-base alloy



United States Patent 3,416,916 DUCTILE COBALT-BASE ALLOY Robert B.Herchenroeder, Kokomo, Ind., assignor to gniokn Carbide Corporation, acorporation of New or No Drawing. Filed July 7, 1966, Ser. No. 563,371 3Claims. (Cl. 75-171) This invention relates to cobalt-base alloys andmore specifically to high-strength, heat-resistant cobalt-base alloyswith superior ductility after aging.

Various cobalt-base alloys containing chromium have been extensivelyused in the past under conditions of high stresses at high temperaturesand it is well known that alloys of this class generally tend to loseductility after prolonged exposure at high temperature. Many attemptshave been made to improve cobalt-base alloys in this respect to therebyreduce or eliminate embrittlement after aging and it can be said that,up to the present, this problem has not been completely solved.

It is an object of this invention therefore to provide a cobalt-basealloy that will remain ductile after prolonged exposure in thetemperature range between about 1400 F. and 1900 1?.

Another object of this invention is to provide a superioroxidation-resistant cobalt-base alloy in the form of wrought products.

A further object of this invention is to provide cobaltbase alloys thathave an optimum combination of high strength and resistance toembrittlement and oxidation. These and other objects were satisfied bythe alloy in accordance with the present invention which COnsistSessentially of about: 18 to 25 percent chromium, 11 to 15 percenttungsten, 1 6 to 25 percent nickel, 0.05 to 0.5 percent carbon, 0.001 to0.025 percent boron balance cobalt and incidental impurities. The alloyof this invention may contain the usual impurities found in commercialalloys of this class, i.e. up to, about percent iron, about 1.5 percentmolybdenum, about 1 percent manganese, about 0.75 percent silicon; andabout 0.15 percent total phosphorous, sulfur, hydrogen, oxygen, andnitrogen. Table 1 shows alloy ranges and specific compositions inaccordance with the present invention.

TABLE 1.ALLOYS OF THIS INVENTION [Composition, in Weight Percent]3,416,!) l 6 Patented Dec. 1 7, 1 968 ICC There are many alloyscommercially available and well known in the art that contain cobalt,chromium, tungsten, and/or molybdenum, nickel and iron and some of thesealloys are shown in Table 2.

These alloys, often called superalloys are especially suitable forservice at high temperature except that they tend to embrittle afterprolonged use at temperatures above about 1400" F. Furthermore, whileeach alloy has certain outstanding individual characteristics, none byitself has the advantage of combined optimum characteristics of highstrength, oxidation resistance and resistance to embrittlement. Forexample, Alloy 1 is known to have outstanding, high temperature strengthbut its oxidation resistance is lower than that of Alloy 2. Alloy 2 hasoutstanding oxidation resistance but its high temperature strength islower than that of Alloy 1. Up to the present such limitations had to betolerated in the application of these superalloys.

The alloys of this invention on the other hand provide an optimumcombination of all the advantageous properties generally associated withthis class of superalloys. As shown hereinbelow, the alloys of thisinvention have strengths equal to or higher than Alloy 1 and also haveoxidation resistance characteristics approaching that of Alloy 2.

Although the exact mechanism of the strengthening effects of theelements proportioned within the scope of this invention is notcompletely understood, it is thought at this time to be a combination ofsolid solution and carbide dispersion hardening.

In accordance with the present invention, chromium, within the rangesshown in Table 1, provides oxidation resistance and contributes to hightemperature strength; chromium contents below the indicated range arenot sufiicient to provide adequate oxidation resistance while chromiumcontents above the indicated range tend to yield alloys of decreaseddutility at room temperature.

Typical Examples Broad Range Preferred Range Alloy A Alloy E Alloy GAlloy H (-238) (-51) (71-2 (65-81-1) Chromium- 18 to 25 18.5 to 22 18. 919. 62 19. 88 21. 89 Tungsten 11 to 15- 11 to 14 11. 4 11.77 13.0 14. 02Carbon .05 to .50-.-" .05 to .35 .25 .15 .30 .11 NickeL 19. 5 19. 2 19.48 Manganeseto 1 46 64 .64 Boron .001 to .025- .001 to 0.02- 008 008.018 O15 Cobalt and (9 Incidental Impurities.

1 Balance.

Tungsten is present in the alloy, within the indicated ranges, as acarbide former and in a solid solution matrix to provide high strength.Molybdenum is not substitutable for tungsten in the alloy of thisinvention, although molybdenum may be present as an unavoidable impurityup to not more than 1.5 percent by weight in the total alloy aspreviously noted.

Carcon is required in the alloy within the range as indicated in Table 1as a solid solution strengthening element and as a carbide former toprovide high strength and nickel must be present in the alloy within theindicated ranges to provide adequate post aging ductility, oxidationresistance, and high temperature strength. Alloys containing less than15.5 percent nickel tend to embrittle after prolonged use attemperatures between about 1400 F. and 1900 F.

Manganese may be present in the alloy in amounts not loys 1 and 2, inaddition Table 6 shows that the alloys of this invention have muchbetter oxidation resistance than alloy 1 of the prior art.

The oxidation tests of Table 6 were identical for all alloys tested. Alloxidation test samples were nominally 0.07-inch thick sheet, 0.75-inchsquare and were uniformly polished to a 120-grit finish. One group ofsamples was exposed at 2000 F. for 100 hours continuously while anothergroup was intermittently exposed at 2000 F. for eight 3-hour periods andfour 19-hour periods for a total of 100 hours as indicated in Table 6.The oxidation rates were determined and are shown on the basis of milspenetration per year (m.p.y.). It is significant that the oxidationrates of the alloys of this invention closely approach those of alloy 2which is generally recognized as the superior wroughtoxidation-resistant alloy presently available in the metals industry.

TABLE 3.COMPOSITION OF TESTED ALLOYS OF THIS INVENTION exceeding aboutone percent and it appears that some metallurgical benefits are providedwith manganese within the range 0.2 to 0.65 percent by weight.

Incidental impurities in amounts normally found in alloys of this type,may be tolerated as previously mentioned while zirconium, columbium,titanium and tantalum, often added in alloys of this class, are notrequired in the alloys of this invention. The combined total content inthe final alloy of zirconium, columbium, titanium and tantalum must notexceed over one percent by weight, as impurities and residuals of priorprocessing steps, in order that the desired combination of properties beobtained.

In the course of experimentation, a series of alloys were prepared by aprocess commonly used in alloys of this class; however, otherconsolidating techniques could be used. In the technique employed, thealloy compositions were melted in an induction furnace, cast intoingots, forged and rolled into 0.063-inch thick sheet for testingpurposes. Prior to testing, the alloys were annealed at 2150 F. for 15minutes and fan cooled. Compositions of alloys prepared and tested aregiven in Table 3. The alloys of this invention described in Table 3 arein the range of about 19-20% Ni, 11-14% W, 19-22% Cr and 0.l-0.3% C. andwill be seen to have an excellent combination of industrially usefulproperties. All of the alloys of Table 3, except alloy B, are inaccordance with this invention. Tensile test results of these alloys,and prior are alloys, are shown in Table 4 while creep andstress-rupture test results are shown in Table 5.

In all tests, the alloys of this invention are seen to be superior, orat least comparable to, both alloys 1 and 2 of the prior art. Forexample, within the range 1400 F. to 1600 F. the average tensilestrengths of the alloys of this invention are about 1.5 times thestrengths of al- TABLE 4.AVERAGE TENSILE PROPERTIES OF ALLOYS TESTEDYield Ultimate Strength Tensile Elongation, 0.2% Strength, PercentOffset, 1,000 1,000 p s l p.s.i.

1 Not determined, specimen broke outside of gage marks.

Total Elong, Lite,

20 percent Hours TABLE 5.CREEP AND STRESS RUPTURE DATA Initial Time forTotal Elonga- Stress, Elong., tlon Percent, Hours Test Temp., F. 1,000p.s.l. percent 1 0 Alloy A:

1 Bend not completed due to failure.

In a further test for ductility after aging, two sample specimens eachof Alloy G of this invention and Alloy 1 in the form described abovewere exposed in air at temperatures and for time periods as vfollows:

Alloy G:

984 hours at 1600 F. 310 hours at 1500 F. Alloy 1:

26 hours at 1600 F. 140 hours at 1500 F.

The sample specimens were given a free bend test, as described above,after aging for the times and temperatures shown above. Both samples ofAlloy G were successfully bent to 180 degrees angle about a radius ofabout inch. Both samples of Alloy 1 fractured before they were bent 90degrees.

What is claimed is:

1. A cobalt base alloy characterized by high temperah, oxidationresistance and resistance to em- DATA [.063-inch thick sheet] Load atCup Depth, Cup Depth,

Rupture, inch mm. 1, 000 p.s.i.

substantially the Le. within about he mini- F. Samples of Alloy A anddescribed in Table 3, were processed under similar except that the AlloyTo obtain best results with the present invention, it

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longed exposure at 1500 Alloy B,

lar conditions. The two alloys are simi brittlement after exposure toelevated temperatures, said alloy consisting essentially of about:

Percent Chromium 18-25 Tungsten -2 11-15 Carbon 0.050.5 Nickel 16-25Boron 0.001-0.025

balance cobalt and incidental impurities.

2. An alloy in accordance with claim 1 wherein:

Is about, percent 3. An alloy in accordance with claim 1 wherein:

Is about, percent Chromium 19-22 Tungsten 11-14 Carbon 0.103 Boron0.0050.018 Nickel 19-20 References Cited UNITED STATES PATENTS 2,744,0105/1956 Callaway 75-17l 2,746,860 5/1956 Binder ct al. 75l71 2,996,3798/1961 Faulkner 75171 3,362,816 1/1968 Winter et al. 75-171 RICHARD O.DEAN, Primary Examiner.

US. Cl. X.R. 14832.5

1. A COBALT BASE ALLOY CHARACTERIZED BY HIGH TEMPERATURE STRENGTH,OXIDATION RESISTANCE AND RESISTANCE TO EMBRITTLEMENT AFTER EXPOSURE TOELEVATED TEMPERATURES, SAID ALLOY CONSISTING ESSENTIALLY OF ABOUT: